Turbine engines commonly operate at efficiencies less than the theoretical maximum because, among other things, losses occur in the flow path as hot compressed gas travels down the length of the turbine engine. One example of a flow path loss is the leakage of hot combustion gases across the tips of the turbine blades where work is not exerted on the turbine blade. This leakage occurs across a space between the tips of the rotating turbine blades and the surrounding stationary structure, such as ring segments that form a ring seal. This spacing is often referred to as the blade tip clearance.
Blade tip clearances cannot be eliminated because, during transient conditions such as during engine startup or part load operation, the rotating parts (blades, rotor, and discs) and stationary parts (outer casing, blade rings, and ring segments) thermally expand at different rates. As a result, blade tip clearances can actually decrease during engine startup until steady state operation is achieved at which point the clearances can increase, thereby reducing the efficiency of the engine.
In a conventional turbine ring segment assembly, the ring segment receives the cooling air through holes in the turbine vane carrier. These holes provide impingement cooling directly on the backside of the ring segment. Because the cooling air is passing through the turbine vane carrier, the turbine vane carrier thermally responds to the cooling air temperature, which results in undesirably large blade tip clearances. Thus, a need exists to reduce this undesirably large blade tip clearance.